1. Field of the Invention
The invention described herein relates to communications systems, and more particularly to packet processing.
2. Background Art
In modem digital communications systems, communicating entities need to have a common, predetermined set of protocols and parameters. Given these protocols and parameters, the entities can communicate in an orderly, efficient manner. Such protocols and parameters are typically implemented according to general functionality. The various functions are often modeled and implemented as multiple layers of a protocol stack. Each layer represents additional protocols that a communicating entity must process, and/or parameters that must be adopted.
One example of a communications system standard protocol is described in the Data Over Cable System Interface Specification (DOCS IS). DOCSIS was originally conceived for cable communications systems. While DOCSIS can be applied to such systems, it is not necessarily limited to cable. Wireless communications systems, for example, can also operate under DOCSIS. Likewise, DOCSIS can be used in satellite communications systems.
DOCSIS can be used in communication systems that include a set of remote communications devices connected to a headend device, such that the headend is responsible for the management of communications both to and from the remotes. The headend is responsible for the distribution of information content to the remotes (the so-called “downstream” direction); in addition, the headend is responsible for management of communications in the other direction, from the remotes to the headend (the “upstream” direction). Generally, in addition to sending content to remotes, the headend issues downstream messages that instruct each remote as to when it can transmit upstream, and what kind of information it can send. In effect, the upstream bandwidth is controlled and allocated by the headend. Any given remote can transmit upstream only after requesting bandwidth and receiving a grant of the bandwidth from the headend. In a time division multiple access (TDMA) environment, bandwidth corresponds to one or more intervals of time. Moreover, the upstream can be organized into a number of channels, with several remotes assigned to each channel. This arrangement allows the headend to manage each upstream communications channel. In this manner, upstream communications are managed so as to maintain order and efficiency and, consequently, an adequate level of service.
In the realm of cable communications, DOCSIS specifies the requirements for interactions between a cable headend and associated remote cable modems. A cable headend is also known as a cable modem termination system (CMTS). DOCSIS consists of a group of specifications that cover operations support systems, management, and data interfaces, as well as network layer, data link layer, and physical layer transport. Note that DOCSIS does not specify an application layer. The DOCSIS specification includes extensive media access layer and physical (PHY) layer upstream parameter control for robustness and adaptability. DOCSIS also provides link layer security with authentication. This prevents theft of service and some assurance of traffic integrity.
The current version of DOCSIS (DOCSIS 1.1) uses a request/grant mechanism for allowing remote devices (such as cable modems) to access upstream bandwidth. DOCSIS 1.1 also allows the provision of different services to different parties who may be tied to a single modem. With respect to the processing of packets, DOCSIS 1.1 allows segmentation of large packets, which simplifies bandwidth allocation. DOCSIS 1.1 also allows for the combining of multiple small packets to increase throughput as necessary. Security features are present through the specification of 56-bit Data Encryption Standard (DES) encryption and decryption, to secure the privacy of a connection. DOCSIS 1.1 also provides for payload header suppression, whereby repetitive ethernet/IP header information can be suppressed for improved bandwidth utilization. DOCSIS 1.1 also supports dynamic channel change. Either or both of the downstream and upstream channels can be changed on the fly. This allows for load balancing of channels, which can improve robustness.
An alternative topology allocates some of the headend functionality to intermediate nodes between the headend and the remote devices. This is known as a distributed CMTS topology. There are several advantages to a distributed CMTS topology. First, relatively few remotes are connected to any given intermediate node. Because an intermediate node has its own PHY and MAC layer processing capability, the amount of bandwidth available to any single remote is increased. Second, a coaxial cable connection extends from a remote to an intermediate node, rather than all the way to a headend. This shorter coaxial cable path allows for higher order modulation methods. For these reasons, among others, a distributed CMTS topology can be preferable over the conventional topology of FIG. 1.
Such a choice, however, requires an appropriate digital communications protocol between the intermediate node and headend. The ethernet protocol is an appropriate solution. It is well understood, and supporting hardware and software are commercially available. A conversion problem remains, however. Packets moving between remote devices and the intermediate nodes are in the DOCSIS format. Communications between intermediate nodes and the headend, however, would use the ethernet protocol, which cannot typically handle a DOCSIS packet. A secondary problem arises in the fact that information stored in the PHY prepend and in the header fields of a DOCSIS packet is needed at the headend for packet classification and other administrative functions. Any conversion must therefore preserve this information.
Hence there is a need for a method and system by which a DOCSIS packet can be converted into a form that can be transmitted and processed using the ethernet protocol. Moreover, such a form would need to retain all the information that characterizes and normally accompanies a DOCSIS packet. This information could then be used by processes further upstream, such as packet classification.